When components are to be coated for various reasons only in local regions, usually the regions of the component that are not to be coated must be covered prior to coating, with a protective layer. For protection of components of titanium alloy from oxidation, DE 4,204,117 discloses applying an oxidation-resistant foil thereon. U.S. Pat. No. 3,876,447 discloses application of a sheet or mat to a substrate.
The type of protection layer applied to the regions not to be coated and the process of application essentially depends on the type of coating to be applied in the local region and in its process of application. For protecting the uncoated regions prior to applying an electrolytic coating thereon, wax layers are applied to the regions not to be coated, as disclosed in DE 4,411,677. Also usable are lacquer layers including photoresist layers. For applying gas-phase coatings in local regions of components of turbine drive units, extremely complex built-up protective layers have been developed. However, the larger the surface to be protected relative to the surface to be coated in the local region, the less economical is the employment of local or regional coating processes of the known type which employ protective layers on the surfaces of the component that are not to be coated.
The coating applied to local areas may also be adversely affected by the configuration of the components, particularly when projecting parts of the component that are not to be coated cover the regions that are to be coated during the coating process, such as vacuum metalizing or spraying whereas for coating processes, such as powder coating or electroplating, the regions to be coated are blocked or obscured so that the coating material may not reach the region to be coated, or the coating material may not be uniformly deposited due to irregularity of distribution of the electrical field. Furthermore, the coatings may be applied more thickly in the coating processes in the exposed areas of the component as compared to areas in which application of the coating is impeded, and particularly due to peaks in field intensity during electroplating or powder deposit. In the case of compound coatings, for example, comprised of a metal matrix containing ceramic, mechanically-resistant particles, there is also the danger that the composition of the coating will not be applied homogeneously. For example, the incorporation of mechanically-resistant particles may not be uniformly distributed and thus can vary in quantity in the coating.
A coating in regions of complex configuration of the component may thus lead to non-uniform thickness of the coating, coating anomalies, and an absence of coating, despite great technical effort with conventional costly covering technologies associated with the corresponding conventional regional deposition or application techniques.